This invention relates generally to preventing corrosion of refractories used in high temperature processes. More particularly, the invention relates to preventing corrosion of silica-based refractories exposed to compounds containing alkali or alkaline-earth metals.
In the manufacturing of glass articles, raw materials are converted at high temperature into a homogeneous melt which can be formed into the articles. Sand is the most common ingredient in glassmaking. Other commonly used glass forming materials are soda ash, calcium limestone, dolomitic limestone, aragonite, feldspar, nepheline, litharge, boron-containing compounds, various fining, coloring and oxidizing agents as well as broken glass also known as cullet. In addition to soda ash (Na2CO3), a number of other alkaline or alkaline-earth compounds are used during glass making. Examples include caustic soda or NaOH and various carbonates such as K2CO3, Li2CO3, MgCO3, CaCO3, SrCO3 and BaCO3.
Melting glass forming materials takes place in a melting unit, for example a melting pot, vessel, tank or furnace. Since the temperatures required to melt glass forming materials are among the highest needed to operate industrial furnaces, special materials are necessary to line the interior surfaces of these glassmelting units.
An additional challenge during glass melting is presented by the corrosive nature of the process itself. Refractory-linings are exposed to both physical and chemical attack from molten glass and from some of the vapors generated during the high temperature melting operation. Particularly corrosive are vapors produced from those glassmaking ingredients containing alkaline and alkaline-earth species.
One advance made in glass melting pertains to directly fired furnaces and involves combustion processes where the conventional oxidant, air, is replaced with pure oxygen or with oxygen enriched air. Oxygen-based combustion results in even higher furnace temperatures. For example, the flue gases produced have temperatures generally exceeding 2000xc2x0 F., typically between 2400xc2x0 F. and 2800xc2x0 F.
Not only are refractories lining these furnaces exposed to higher temperatures, but also oxygen-based combustion influences the corrosion effects attributed to compounds containing alkali and alkaline-earth metals. The relatively low gas velocities present in oxygen-fired furnaces slows the mass transfer rate, leading to a reduction in the amount of species volatilized from the glass melt and due to the significant reduction in nitrogen, the partial pressure of all components present in the furnace atmosphere increases and the concentration of vapors containing alkaline or alkaline-earth species can be as much as three to four times higher when compared to concentrations found in conventional air-fired furnaces.
Many existing furnaces employ silica-based refractories. Although materials that can better withstand corrosion are available, such as, for example, zirconia-based refractories, replacing conventional silica linings with more advanced materials imposes significant capital expenses and often requires modifications in the furnace structure, since nobler refractories tend to have a higher density than those containing silica.
There continues to be a need, therefore, to prevent, control or reduce the effects of corrosion observed in glass melting furnaces lined with conventional silica-based refractory materials, especially when such furnaces are operated under oxy-fuel conditions.
Accordingly, it is an object of the invention to provide silica-based refractories having improved corrosion resistance.
The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A process for producing silica-based refractory having improved corrosion resistance comprising:
(A) providing a silica-based refractory;
(B) impregnating the silica-based refractory with a precursor capable of being converted into a protective material; and
(C) providing energy to the silica-based refractory impregnated with said precursor and converting said precursor to said protective material thereby producing a silica-based refractory impregnated with said protective material having improved corrosion resistance.
Another aspect of the invention is:
A silica-based refractory article having improved corrosion resistance comprising:
(A) a silica refractory matrix; and
(B) a protective material impregnated onto the silica refractory matrix.
As used herein the term xe2x80x9cprotective materialxe2x80x9d means a chemical compound other than silica which is more resistant to attack by corrosive compounds than is the refractory itself. For example, a protective material may be inert or less reactive in the corrosive environment of a particular application than the unprotected refractory; or, if the protective material does react with the corrosive species and/or with the refractory matrix, the rate of mass loss and wear observed for refractories impregnated with the protective material is lower than that exhibited, under similar conditions, by the unprotected refractory.
As used herein the terms xe2x80x9cimpregnatexe2x80x9d or xe2x80x9cimpregnationxe2x80x9d of a refractory or a refractory matrix means the penetration into pores, crevices, surface irregularities and/or imperfections of a silica-based refractory (matrix) and occupying at least a fraction of the total cavity volume of the pores, crevices, irregularities and/or imperfections.
As used herein the term xe2x80x9cprecursor solutionxe2x80x9d means precursors dissolved, dispersed, suspended or emulsified in a liquid medium.
As used herein the terms xe2x80x9calkalixe2x80x9d or xe2x80x9calkalinexe2x80x9d means metals of the first main group of the periodic table, in particular lithium, sodium and potassium.
As used herein the term xe2x80x9calkaline-earthxe2x80x9d means elements of the second main group of the periodic table, especially beryllium, magnesium, calcium, strontium, and barium.